Droplet-on-demand ink jet printing systems eject ink droplets from print head nozzles in response to pressure pulses generated within the print head by either piezoelectric devices or thermal transducers, such as resistors. The ejected ink droplets are propelled towards an image receiving member where each ink droplet forms a spot or pixel on the image receiving member to produce an image. The print heads have droplet ejecting nozzles and a plurality of ink channels, usually one channel for each nozzle. The channels couple the nozzles to an ink reservoir in the print head to supply ink to the nozzles.
In a typical piezoelectric ink jet printing system, the pressure pulses that eject liquid ink droplets are produced by applying an electric pulse to the piezoelectric devices, one of which is typically located within each one of the ink channels. Each piezoelectric device is individually addressable to enable an electric pulse or firing signal to be generated and delivered to particular piezoelectric devices in a print head. The firing signal causes the piezoelectric device receiving the signal to bend or deform and pressurize a volume of liquid ink adjacent the piezoelectric device. As a voltage pulse is applied to a selected piezoelectric device, a quantity of ink is displaced from the ink channel and a droplet of ink is mechanically ejected from the nozzle associated with the selected piezoelectric device. The ejected droplets are propelled towards an image receiving member to form an image on the image receiving member. The respective channels from which the ink droplets were ejected are refilled by capillary action from an ink supply.
In some printers, commonly referred to as direct printers, the image receiving member is a sheet or web of receiving medium, such as paper. In other printers, commonly known as offset or indirect printers, the image receiving member is a rotating drum or belt coated with a release agent. The print head ejects droplets of melted ink onto a thin film of release agent coating the rotating image receiving member to form an image. This image is then transferred to a recording medium, such as a paper sheet. The release agent helps facilitate the transfer of the image because the image is really formed on the layer of release agent so the image does not affix to the image receiving member. The transfer is generally conducted in a transfixing nip formed by the rotating image receiving member and a rotating pressure roll, which is also called a transfix roll. The transfix roll may be heated or the recording medium may be pre-heated prior to entry in the transfixing nip. As a sheet of paper is transported through the nip, the fully formed image is transferred from the image receiving member to the sheet of paper and concurrently fixed thereon. This technique of using heat and pressure at a nip to transfer and fix an image to a recording medium passing through the nip is typically known as “transfixing,” a well known term in the art.
In some print head systems known to the art, each ink droplet has a mass of approximately 20 nanograms (ng). Some newer print head systems eject droplets with a smaller mass, typically of about 10 ng or less. These small droplets form smaller pixels on the image receiving member, which has the advantage of generating finer resolution images where more pixels may be placed in a given surface area. Another advantage is that a printing system that uses smaller droplets may position each droplet deposited on the image receiving member with greater precision.
The use of small droplets in indirect printing systems does have the drawback that these small droplets may result in “image dropout” during the transfixing process. Image dropout occurs when an ink drop remains on the image receiving member rather than being transferred to the recording medium. Image dropout may produce an image in which partial or missing drops are noticeable in the image on the recording medium after transfixing is completed. In indirect printing systems these small droplets do not transfer from the image receiving member as well as larger drops do, resulting in image dropout. The image dropout is exacerbated when a rougher-surfaced medium like recycled paper passes through the nip.
In an effort to reduce dropout, present indirect printers with pixels formed from small droplets may pass print media through the nip at a slower rate of speed, but this negatively affects a printer's throughput efficiency. A printing system that utilizes print heads that produce small droplets, while also avoiding image dropout or the need to reduce the throughput of print media, benefits the field.